The present invention relates to a method for the preparation of silicone resins. More particularly, the present invention relates to a method for the preparation of silicon-bonded hydrogen (SiH)--containing silicone resins.
Silicone resins, which may be liquid or solid at ambient temperature, have long been known and at present are used in numerous applications, for example, as electrical-insulating varnishes, heat-resistant coatings, and sealants for semiconductor elements. Silicone resins may be classified into several categories as a function of the particular combination of siloxane units. Already well known are the MQ silicone resins, which consist of monofunctional siloxane unit R.sub.3 SiO.sub.1/2 (M unit) and tetrafunctional siloxane unit SiO.sub.4/2 (Q unit), and the MDQ silicone resins, which consist of M, Q, and difunctional siloxane unit R.sub.2 SiO.sub.2/2 (D unit).
Methods known for the preparation of these silicone reasons have been described. For example, Daudt et al. in U.S. Pat. No. 2,676,182 and Currie et al. in U.S. Pat. No. 2,814,601 teach that silicone resins can be prepared by converting a water-soluble silicate (such as water glass or sodium orthosilicate) into a silicic acid monomer or silicic acid oligomer using, for example, hydrochloric acid or sulfuric acid, and, when an adequate polymerization has been achieved, trapping with trimethylchlorosilane to yield the MQ silicone resin.
Goodwin, in U.S. Pat. No. 2,857,356, discloses a method for the preparation of a silicone resin by the cohydrolysis of an alkyl silicate and a hydrolyzable trialkylsilane mixture through the addition of water to yield an MQ silicone resin.
A problem associated with the MQ silicone resins synthesized by Daudt et al. and Currie et al. above is the large amount of silanol that remains in their resins. When the method of Goodwin is employed by using a trialkylalkoxysilane as the hydrolyzable trialkylsilane, silanol and large amounts of alkoxy groups remain in the synthesized MQ silicone resin. As a result, when the synthesis of a SiH-containing MQ silicone resin is attempted by these methods through the addition of dimethylchlorosilane (Me.sub.2 HSiCl) or methyldichlorosilane (MeHSiCl.sub.2), the residual silanol and residual alkoxy react with the silicon-bonded hydrogen (Si--H), which prevents quantitative introduction of the Si--H group. Moreover, even after the successful introduction of the Si--H group, the residual silanol and residual alkoxy in the silicone resin slowly react with the Si--H, thereby creating the problem of poor storage stability. An additional problem with the cited methods is the difficulty in using them to prepare high-molecular-weight SiH-containing silicon resins.
Shirahate, in U.S. Pat. No. 4,707,531, discloses a method for producing an MQ resin by dripping an alkyl silicate (alkyl orthosilicate or a partial hydrolysis condensate of alkyl orthosilicate) into a mixture of aqueous hydrochloric acid which contains at least 5% hydrogen chloride and a trialkylsilane or a disiloxane or a mixture thereof, at a temperature of from 0.degree. to 90.degree. C. with stirring. This method is problematic from the standpoint of environmental pollution because it generates an aqueous hydrochloric acid solution containing large amounts of methanol as an effluent. It is also difficult to apply this method to the preparation of high-molecular-weight SiH-containing silicone resin.